M3 Glacial - Interglacial Cycles

Topic

Glacial-interglacial cycles represent the mode of climate variability with the greatest amplitude throughout the Quaternary—the period covering the last 2.6 million years of Earth's history. Between a glacial and an interglacial period, global temperature fluctuates by four to eight degrees, and carbon dioxide concentration ranges from 180 to 280 parts per million.

Milankovitch orbital forcing acts as the initial trigger for these glacial-interglacial cycles: Earth's orbital eccentricity (with a period of approximately 100,000 years), axial tilt or obliquity (approx. 41,000 years), and axial precession (approx. 23,000 years) collectively modulate the amount of seasonal insolation received by high latitudes in the Northern Hemisphere. However, Milankovitch orbital forcing alone is insufficient to explain the full amplitude of these cycles; the climate system amplifies this forcing through internal feedbacks involving carbon dioxide, ice albedo, water vapor, and changes in ocean circulation.

The insolation received by high Northern Hemisphere latitudes at any given time is calculated based on latitude, season, and Milankovitch orbital parameters; however, the climate system's response to this insolation is non-linear. There is a marked asymmetry between slow glaciation—taking nearly 100,000 years to develop—and rapid deglaciation, which occurs in just 10,000 years; this asymmetry cannot be explained solely by Milankovitch orbital forcing. Energy balance models incorporating an interactive carbon cycle reproduce this asymmetry by introducing non-linear thresholds that are absent in Milankovitch orbital forcing alone. The volume of ice accumulated during glacial-interglacial cycles roughly tracks summer insolation measured at 65 degrees north latitude, yet it lags behind that insolation by several thousand years.

An unresolved issue is that the ~100,000-year eccentricity period dominates the most recent glacial-interglacial cycles, even though eccentricity is the weakest of the three Milankovitch orbital forcing components described earlier; this is known as the "100,000-year problem" and remains a subject of active research. Three mechanisms have been proposed to explain this problem: a mechanism in which random climate noise synchronizes with and amplifies the weak eccentricity forcing far beyond what Milankovitch forcing alone would produce; a non-linear role played by the North American ice sheet, whose internal dynamics could introduce the ~100,000-year period independently of Milankovitch forcing; and changes in the carbon cycle, evident in the methane and carbon dioxide cycles recorded in ancient ice.

Two primary paleoclimatic records allow for the testing of these glacial-interglacial cycle models: an Antarctic ice core spanning the last 800,000 years of ice accumulated layer by layer, and marine sediments recovered from the ocean floor by various ocean drilling programs. Both Antarctic ice cores and marine sediments preserve—each in its own way—the same sequence of glacial-interglacial cycles triggered by Milankovitch orbital forcing and amplified by the climate system's internal feedbacks; it is this very sequence that ultimately makes it possible to distinguish, among the mechanisms proposed to explain the "100,000-year problem," which ones are consistent with what the paleoclimatic record actually shows.

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